1. Field of the Invention
The present invention relates to a dispersion compensating fiber and an optical transmission system including the same, which are applied to an optical fiber transmission network capable of long-distance and high-bit-rate optical communication utilizing the 1.55 xcexcm-band wavelength-multiplexing signal light.
2. Related Background Art
From social needs based on the coming of advanced information society, research and development has been conducted vividly heretofore as to high-bit-rate high-speed communication such as video communication and long-distance communication such as international communication utilizing the optical fiber transmission network.
In the case of the optical fiber transmission network to realize such long-distance and high-bit-rate optical communication, first, its transmission lines need to be optical fibers that permit only single-mode propagation. It is because mode dispersion (represented by dispersion due to a difference between group velocities of respective propagation modes) inevitably takes place in the case of multimode communication.
Thus, the first countermeasure was the single-mode optical fiber permitting only single-mode propagation. This single-mode optical fiber is free of occurrence of mode dispersion, but chromatic dispersion represented by the sum of material dispersion (dispersion due to wavelength dependence of refractive index specific to a material of optical fiber) and structural dispersion (dispersion due to wavelength dependence of group velocity of propagation mode) confines transmission capacity. Specifically, even if the wavelength of light emitted from a light source is said to be single, though rigorously speaking, it will have a certain spectral width. When a light pulse having this spectral width propagates in the single-mode optical fiber having predetermined chromatic dispersion characteristics, the width of the light pulse if broadened, so as to deform the pulse shape. This chromatic dispersion is expressed as a transmission delay time difference per unit spectral width (nm) and unit optical fiber length (km) in units of ps/km/nm).
It is, however, known that silica normally used as a material for optical fiber shows zero material dispersion near the wavelengths of 1.26 to 1.29 xcexcm. Since the structural dispersion varies depending upon parameters of optical fiber, the optimum design of the parameters of optical fiber permits the material dispersion and the structural dispersion to cancel each other near the wavelengths of 1.3 to 1.32 xcexcm, thereby realizing zero chromatic dispersion. Therefore, use of single-mode optical fiber allows longer-distance and larger-bit-rate optical communication near the wavelength 1.3 xcexcm than use of multimode optical fiber does. In practice, the single-mode optical fibers are used in optical communication of the communication distance of several hundred km and the communication capacity of several hundred Mbit/sec.
However, transmission loss of optical fiber is minimum in the 1.55 xcexcm wavelength band, from which there have been desires for optical communication utilizing the 1.55 xcexcm-band light. This resulted in developing a dispersion shifted fiber in which the wavelength where the chromatic dispersion was zero (zero-dispersion wavelength) was shifted into this wavelength band. In the dispersion shifted fiber, because the material dispersion cannot be changed so much, the index profile thereof is designed optimally to change the value of structural dispersion, thereby setting the zero-dispersion wavelength in the vicinity of 1.55 xcexcm. This dispersion shifted fiber, together with an erbium (Er)-doped optical fiber amplifier, is employed in the long distance optical fiber transmission system with the transmission capacity being several Gbit/sec, utilizing the 1.55 xcexcm-band wavelength division multiplexing (WDM) signal light.
On the other hand, there are many single-mode optical fibers already installed heretofore. Therefore, needs exist for optical communication in the 1.55 xcexcm wavelength band utilizing the existing single-mode optical fiber transmission network. Thus, an attempt has been made to cascade-connect connect a dispersion compensating fiber having negative chromatic dispersion and negative dispersion slope to a single-mode optical fiber having positive chromatic dispersion in the 1.55 xcexcm wavelength band, thereby canceling out the chromatic dispersion and dispersion slope as the whole of optical transmission line (for example, as in the bulletin of Japanese Laid-open Patent Application No. 6-11620).
In a graph to show the chromatic dispersion, the dispersion slope is given as a slope of the graph.
The inventors investigated the above-stated prior art and found the following problems. Specifically, with the above-stated dispersion shifted fiber, the chromatic dispersion thereof becomes zero at a predetermined wavelength near the wavelength 1.55 xcexcm. However, the chromatic dispersion is not zero in the regions before and after the wavelength (the zero-dispersion wavelength) and the chromatic dispersion increases with increasing wavelength in general when a sign of chromatic dispersion is positive. In other words, the dispersion slope (which is the wavelength dependence of chromatic dispersion and is expressed in units of (ps/km/nm2)) has a positive sign in this condition. This would be a problem in the case of communication by the wavelength division multiplexing (WDM) system for multiplexing signal light components of mutually different wavelengths in order to further raise the transmission speed to higher rates. Namely, there is such a tendency that among the 1.55 xcexcm-band wavelength-multiplexing signal light (having a plurality of wavelengths) the chromatic dispersion becomes larger (positive) for signal light components of longer wavelengths while the chromatic dispersion becomes smaller (negative) for signal light components of shorter wavelengths (i.e., there is such a trend as to have positive dispersion slope), which results in the limit of increase in transmission speed in the WDM method.
On the other hand, studies on dispersion-flattened optical fibers the both chromatic dispersion and dispersion slope of which become nearly zero in the 1.55 xcexcm wavelength band are reported, for example, in Kubo et al., xe2x80x9cCharacteristics of double cladding type low-dispersion SM fiber,xe2x80x9d C-374, Abstracts (The spring meeting, 1990); Institute of Electronics, Information and Communication Engineers of Japan, and P. K. Bachmann et al., xe2x80x9cDispersion-Flattened Single-Mode Fibers Prepared with PCVD: Performance, Limitations, Design Optimization, xe2x80x9d J. of Lightwave Technol., Vol. LT-4, No. 2, pp. 858-863 (1986). However, the dispersion=flattened fibers need to be fabricated with extremely precise control of the size, such as the core diameter, and the refractive index profile and are hard to fabricate, thus not coming to the stage of practical application yet.
A dispersion compensating optical fiber according to the present invention is optically connected to the conventional optical fiber (an optical transmission line being a compensated object), so as to compose an optical transmission system. It is, therefore, an object of the present invention to provide a dispersion compensating fiber enabling the long-distance and high-bit-rate optical communication by optically connecting the dispersion compensating fiber according to the present invention to the conventional optical fiber transmission line in respectively appropriate lengths, thereby improving the overall chromatic dispersion and dispersion slope of the optical transmission line in the 1.55 xcexcm wavelength band (i.e., making absolute values of chromatic dispersion and dispersion slope closer to zero), and to provide an optical transmission system comprising it.
The dispersion compensating fiber according to the present invention is used for compensated objects mainly including dispersion shifted fibers the zero-dispersion wave-length of which is set in the range of 1450 to 1650 nm and optical fiber transmission lines including such dispersion shifted fibers. Further, the dispersion compensating fiber according to the present invention is preferably used for compensated objects including dispersion shifted fibers the zero-dispersion wavelength of which is set in the range of 1450 to 1550 nm and optical fiber transmission lines including such dispersion shifted fibers. Either one of these dispersion-compensated objects has positive dispersion slope.
Accordingly, the dispersion compensating fiber according to the present invention is characterized by having the following characteristics for 1.55 xcexcm-band light: chromatic dispersion not less than xe2x88x9240 ps/km/nm and not more than 0 ps/km/nm; dispersion slope not less than xe2x88x920.5 ps/km/nm2 and not more than xe2x88x920.1 ps/km/nm2; transmission loss not more than 0.5 dB/km; polarization mode dispersion not more than 0.7 ps.kmxe2x88x92xc2xd; mode field diameter not less than 4.5 xcexcm and not more than 6.5 xcexcm; cut-off wavelength not less than 0.7 xcexcm and not more than 1.7 xcexcm at the length of 2 m or the like; and bending loss at the diameter of 20 mm, not more than 100 dB/m.
In this specification, xe2x80x9c1.55 xcexcm wavelength bandxe2x80x9d means the band is in the range of wavelengths 1500 to 1600 nm.
The transmission line can be improved in the overall chromatic dispersion and dispersion slope in the 1.55 xcexcm band by optically connecting the dispersion compensating fiber with an optical fiber as a compensated object (mainly, a dispersion shifted fiber of a transmission system including this dispersion shifted fiber) at a predetermined ratio of lengths. Further, long-distance and high-bit-rate optical communication becomes possible based on these characteristics and the conditions of transmission loss, polarization mode dispersion, mode field diameter, cut-off wavelength (cut-off wavelength in the reference length of 2 m), and bending loss (bending loss at the diameter of 20 mm).
Further, the dispersion compensating fiber according to the present invention preferably has such characteristics for the 1.55 xcexcm band light that the chromatic dispersion thereof is not less than xe2x88x9220 ps/km/nm and not more than xe2x88x925 ps/km/nm and that the dispersion slope thereof is not less than xe2x88x920.4 ps/km/nm2 and not more than xe2x88x920.13 ps/km/nm2. This setting of chromatic dispersion and dispersion slope allows the whole of the optical transmission system including the dispersion compensating fiber (and including the dispersion shifted fiber the zero-dispersion wavelength of which is set in the wavelength range of 1450 to 1650 nm, preferably in the range of 1450 to 1550 nm) to be compensated more suitably (which means that the absolute values of chromatic dispersion and dispersion slope of the whole can be made closer to zero).
For achieving the above characteristics, the dispersion compensating fiber according to the present invention is characterized by being a single-mode optical fiber mainly containing a silica-based glass, which comprises at least: a core region having a predetermined refractive index, said core region having an outer diameter not less than 3.5 xcexcm and not more than 6.0 xcexcm; an inside cladding region provided on the periphery of the core region and having a lower refractive index than the core region, wherein a ratio of the outer diameter of the core region to an outer diameter of this inside cladding region is not less than 0.3 and not more than 0.5; and an outside cladding region provided on the periphery of the inside cladding region and having a higher refractive index than the inside cladding region and a lower refractive index than the core region, wherein a relative refractive index difference between the outside cladding region and a portion with a maximum refractive index in said core region is not less than 0.6% and not more than 1.4% and wherein a relative refractive index difference between the outside cladding region and a portion with a minimum refractive index in the inside cladding region is not less than 0.25% and not more than 0.65%.
Further, when the dispersion compensating fiber is of triple cladding structure, the dispersion compensating fiber has an intermediate cladding region having a higher refractive index than the outside cladding region and a lower refractive index than the core region, between the above inside cladding region and outside cladding region. A refractive index difference between the outside cladding region and a portion with a maximum refractive index in the intermediate cladding region is not less than 0.2% and not more than 0.5%.
In order to attain the sufficient relative refractive index difference with a low dopant concentration, the dispersion compensating fiber according to the present invention, having the above configuration, is preferably made in such a manner that the above core region is doped with the germanium element and the above inside cladding region is doped with the fluorine element. In addition, it is also possible to realize such a configuration that the above outside cladding region is also doped with the fluorine element.
Further, the dispersion compensating fiber according to the present invention, together with another optical fiber (compensated object) optically connected to the dispersion compensating fiber and forming a part of the optical transmission line, constitutes an optical transmission system (see FIG. 1). The optical transmission system including the dispersion compensating fiber preferably has the dispersion slope not less than xe2x88x920.02 ps/km/nm2 and not more than 0.05 ps/km/nm2 for the 1.5 xcexcm-band light. Such an optical transmission system permits long-distance and high-bit-rate optical transmission and particularly, in realizing the optical communication utilizing multi-wavelength light by the WDM method, it permits much longer-distance and higher-bit-rate optical communication.
The optical fiber transmission line as a dispersion-compensated object, forming the optical transmission line of the optical transmission system together with the dispersion compensating fiber, is preferably a dispersion shifted fiber the zero-dispersion wavelength of which is shifted to 1560 nm or less. When the compensated object is the dispersion shifted fiber having the zero-dispersion wavelength not more than 1.56 xcexcm, the dispersion shifted fiber is readily compensated for the chromatic dispersion and chromatic dispersion slope by the dispersion compensating fiber according to the present invention.
In addition, the optical transmission system comprising the dispersion compensating fiber and the dispersion shifted fiber as a compensated object as described above may further comprise an optical fiber amplifier forming a part of the optical transmission line. This optical fiber amplifier comprises at least an optical fiber for amplification a core region of which is doped with the erbium element, an excitation light source for outputting exciting light for exciting the erbium element in the optical fiber, to the optical fiber, and an optical coupler for optically coupling the excitation light source with the optical fiber. Since the length of the optical fiber for amplification inserted in this optical transmission system is far shorter than the length of the dispersion shifted fiber or the whole optical transmission line including the dispersion shifted fiber, contribution thereof to the chromatic dispersion and dispersion slope to be compensated for as the whole of optical transmission line is negligible.
On the other hand, the dispersion compensating fiber according to the present invention may be made in such a configuration that the core region is doped with the erbium element. The dispersion compensating fiber containing the erbium element as described can function as an optical fiber for amplification.
Accordingly, the optical transmission system comprising the dispersion compensating fiber the core region of which is doped with the erbium element comprises the dispersion compensating fiber according to the present invention, another optical fiber (compensated object) optically connected to the dispersion compensating fiber and forming a part of the optical transmission line, an excitation light source for outputting exciting light for exciting the erbium element in the dispersion compensating fiber, to the dispersion compensating fiber, and an optical coupler for optically coupling the excitation light source with the dispersion compensating fiber. According to this configuration, the optical transmission system comprising the dispersion compensating fiber, as the whole of optical transmission line, has the dispersion slope not less than xe2x88x920.02 ps/km/nm2 and not more than 0.05 ps/km/nm2 for the 1.5 xcexcm-band light. The optical transmission system of this type enables longer-distance, higher-bit-rate, and low-loss optical communication.
In the optical transmission system comprising this optical fiber amplifier (having the dispersion compensating fiber according to the present invention), the above dispersion-compensated object is preferably a dispersion shifted fiber the zero-dispersion wavelength of which is shifted to 1560 nm or less.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.